Tubular spring element and a method of producing the same

ABSTRACT

A tubular spring having a first end and a second end is fixed at its first end in a plastic holder. The plastic holder has a central passage. The second end is closed by a terminal fitting. The interior of the tubular spring communicates with the passage in the holder but is otherwise sealed in relation to the outside atmosphere. The tubular spring is fixed within the holder by injection molding the holder about the first end of the spring. An insert may be disposed within the portion of the tubular spring within the holder to prevent deformation of the spring. Alternatively, a removable filler material may be introduced into the tubular spring and removed after the injection molding step.

United States Patent [1 1 Sellmaier et a1.

111 3,757,582 Sept. 11., 1973 Germany 22 Filed: Feb. 1, 1972 [21] Appl.No.: 224,112

[30] Foreign Application Priority Data Aug. 10, 1971 Germany P 21 40086.2

52] us. Cl. 73 /418 [51] Int. Cl. G011 7/04 [58] Field of Search 73/418,41 1, 412,

[56] References Cited UNITED STATES PATENTS 2,934,729 3/1960 Bourns73/411 TUBULAR SPRING ELEMENT AND A METHOD'OF PRODUCING THE SAMEInventors: Englebert Sellmaier, 8242 Bischofseiwsen, Loipler; Wolfgangllartling, 824 Schonau, l-laus Waltraud; Helmut Schnurr, 8242Bischofswiesen, Rosspointweg, all of 3,527,102 9/1970 Harland 73/416Primary Examiner-Donald O. Woodiel Att0mey.lordan B. Bierman et al.

[57] ABSTRACT A tubular spring having a first end and a second end isfixed at its first; end in a plastic holder. The plastic holder has acentral passage. The second end is closed by a terminal fitting. Theinterior of the tubular spring communicates with the passage in theholder but is otherwise sealed in relation tov the outside atmosphere.

The tubular spring is fixed within the holder by injection molding theholder about the first end of the spring. An insert may be disposedwithin the portion of the tubular spring within the holder to preventdeformation of the spring. Alternatively, a removable filler materialmay be introduced into the tubular spring and removed after theinjection molding step.

25 Claims, 12 Drawing Figures Patented Sept. 11, 1973 3 Sheets-Sheet 2Patented Sept. H, 1973 3 Sheets-Sheet 5 4'4 Q 3 5 /2 E 2 YfliTUBULARSPRING ELEMENT AND'AMETHOD F PRODUCING THE SAME This inventionrelates to a tubular spring element, as used in measuring instruments,such as, more particularly, pressure gauges, hydrometers, barometers orthermometers.

In such measuring instruments the element which responds to the measuredquantity, viz. the pressure or temperature, usually consists of aso-called bourdon tube which is a roughly circularly arched tubularspring of approximately oval cross-section, one end of which is securedin a holder, and the other end of which is sealed by a terminal fitting.The interior of the tubular spring element communicates with a passagethrough the holder, but is otherwise sealed in relation to the outsideatmosphere. The pressure of a fluid entering the interior of the tubularspring element through the holder, or the pressure generated by atemperature change, causes the tube, which is arched in position ofrest, to somewhat straighten out, the resultant deflection of its freeend, depending upon the magnitude of the quantity that is to bemeasured, being indicated for example by a pointer on a scale. Insteadof a bourdon tube the measuring element may also have the form of atubular element which is helically or spirally coiled or bent in someother way, and which may have a round, oval orother cross-section.-

In conventional bourd'on tubes the tubular spring element usuallyconsists of a brass or bronze strip. The fixed end issoldered into aholder made of brass, stainless steel or some other metal. The terminalfitting at the other end of the tube likewise consists of a solderedpart made of brass, stainless steel or some other metal. This known formof construction of bourdon tubes has the following drawbacks. The metalused for the holder and the terminal fitting is relatively expensive andheavy. The holder isusually made of a blank cut off a bar section and,in addition to the drilling of the internal passage ,'a slit for fittingthe tube, external threads and other holesfor the aftixation of theindicating linkage must be provided and other machining operationsconsecutivelyperformed. The production of the holder thereforetakestime, Another very important drawback of conventional bourdon tubes isthat the heat generated when the tube is soldered into the holder andwhen the terminal member is soldered to the "other end ofthe tubeaffects thecharacteristics of thefbourdon tube in an uncontrollablemanner. Owing to these unpredictable effects each bourdon tubemanufactured in a major production run or by mass production methodswill therefore exhibit slightly differing measuring characteristics.Each element must therefore be separately calibrated, a necessity whichit is clearly desirable to, avoid, particularly in a massproducedarticle. Yet-another drawback of the conventional arrangement is thatitis practically impossible to attachthe holderandthe terminal fittingto the ends of the bourdon tube bysoldering, so that the relationbetween the parts isalways exactly the same. Again, this circumstancemakes subsequent readjustment of each element unavoidable.

It is the object of the present invention to eliminate theabove-mentioned short-comings and to provide anim'proved tubular: springelement which is substantially lighter in weight, and which can beproduced at much lower cost with far greater precision than aconventional element.

According to the present invention, a tubular spring element of thespecified kind comprises a tubular spring, one end of which is fixed ina holder, and the other end of which is closed by a terminal fitting,the interior of the tubular spring communicating with a passageextending through the holder, but being otherwise sealed in relation tothe outside atmosphere, and the holder (and preferably also the terminalfitting) is made of a synthetic plastic material, preferably by in- 0jection molding around the respective end of the tubular spring.

In widely different fields of technology it is already a well known stepto substitute plastics for previously usedtraditional materials.However, in the case of bourdon tubes and like tubular spring elementsthis has never been done, because the embedment of the ends of the tubein a plastic material, particularly by injection molding the plasticaround the ends of the tube, results in so much deformation andcompressional damage that unless special precautions are taken, thetubes become useless, especially in the case of the light weight bourdontubes that are needed for measuring lower pressures.

In a preferred form of the invention the end of the tubular springaround which the plastic holder is injection molded is provided withmeans which protect the tubular element against deformation whilst themolding process takes place, and which preferably additionally alsoserves to secure the tube in the holder. Such means may consist of arigid insert having a longitudinal bore, which is fitted into theappropriate end of the tube.

Preferably, this rigid insert extends inside the tube be In anotherarrangement the said additional means may consist in introducing intothe end of the tubular spring, before it is embeddedin the injectionmolding, a filler, such as wax, salt, washing powder or some othermeltable or soluble medium which can be melted or dissolved out when theinjection molding operation has been completed.

With advantage the holder and the terminal fitting may be moldedsimultaneously around the respective ends of the tubular spring in thesame mold. This ensures that the parts attached to the ends of thetubular spring between which the measuring system is disposed willconstitute firmly located reference points.

Other details and advantages of the invention will be understood as thefollowing particular description of preferred embodiments shown in thedrawings proceeds. In the drawings,

FIG. 1 is an elevation of a bourdon tube shown in association with alinkage, pointer and scale to illustrate its principle ofoperation;

FIG. 2 shows the bourdon tube holder and part of the inserted tube, insection on a larger scale;

FIG. 3 shows in section part of a different embodiment of the holder andof the bourdon tube;

FIG. 4 is a section of the bourdon tube taken on the line lVlV in FIG.3;

FIGS. 5 to 7 are other variants of the part illustrated in FIG. 2;

FIG. 8 is a section of the embodiment according to FIG. 7, taken on theline VIII-Vlll;

FIG. 9 is a detail view of a modified insert to be fitted into one endof the bourdon tube; and

FIGS. 10 to 12 are views similar to FIG. 2 showing other ways of fixingthe end of the bourdon tube in the holder.

Referring to FIG. 1, a tubular spring element which here has the form ofa bourdon tube comprises a bourdon tube 20 proper, one end of which isfixed in a holder 22. The other end is scaled by a terminal fitting 24.A passage 26 which extends through the holder 22 communicates with theinterior of the bourdon tube 20. If the bourdon tube 20 which in theillustrated embodiment is roughly circularly arched and normally has anoval cross-section is subjected to a pressure or a temperature increase,it will straighten to a given extent, causing the terminal fitting 24 tobe deflected in the upward direction, as seen in FIG. 1. This deflectionis transmitted by a linkage 28 to an indicating head 30, which isattached for instance by screw means 31 to the upper end of the holder22. The measured quantity is indicated by a pointer 32 which pivots inthe indicating head 30 and which moves across a calibrated scale 34.

The bourdon gauge so far described including the linkage, the indicatinghead and the pointer, is well known in the art. The bourdon tube 20 mayhave a circular, oval, lenticular, polygonal or other corss-section andit may be arcuate as shown, or curved in the form of a spiral or ahelix, or bent in some other way in one or more planes. The bourdon tubeitself is made of metal, usually of brass or bronze strip. When used asa pressure gauge a bourdon tube can be used for measuring pressures fromless than one kilopond/sq.cm to several thousand kiloponds/sq.cm.

The holder 22 and preferably the terminal fitting 24 are made of asynthetic plastic material and both these parts 22 and 24 are injectionmolded around the respective ends of the bourdon tube. Injection moldingis nowadays done at pressures between about 60 and 90 kilopond/sq.cm. Inorder to protect the bourdon tubes, particularly more delicate bourdontubes, against deformation whilst the holder is being molded, a suitableinsert is first introduced into the appropriate end of the tube. Thisinsert may have various shapes and, if suitably disposed and/orappropriately designed, it may also assist in fixing the bourdon tube inthe holder. A few possible forms of construction of such an insert 36are illustrated in FIGS. 2 to 6, 9 and 10. The crosssection of theinsert 36 approximately conforms to the internal cross-section of thebourdon tube 20.

In FIG. 2 the insert 36 has a central axial bore 38 which is closed atone end by the holder 22 and traverses a substantial part of the lengthof the insert. The circumferential surface of the insert 36 is providedwith one or more peripherally and/or axially distributed dimples 40 (cf.also FIGS. 9 and 10). Instead of dimples 40, one or more annular groovescould be provided.

The insert 36 is inserted into the smooth end of the previously bentbourdon tube, in such a way that the open end of the axial bore 38communicates with the interior of the bourdon tube 20. The insert 36 maybe additionally secured inside the end of the bourdon tube by means ofan adhesive, or by being soldered or welded or by some other bond. Theend of the bourdon tube 20 containing the insert 36 is then introducedinto an injection mold (not shown). The axial length of the insert 36and its disposition inside the bourdon tube 20 are so chosen that thetube cannot be deformed by the surrounding injection mold, i.e. that theinsert 36 extends inside the bourdon tube 20 to a point beyond theexterior of the holder that is to be molded. The mold is so designedthat threads 42 for securing the holder 22, and with it the entirebourdon gauge are formed on the bottom end of the holder, as we'll'asholes 44 in the upper end of the holder for the reception of fasteningelements such as screws 31. During the injection molding operation, thepressure of the synthetic plastic material forces the metal wall of thebourdon tube 20 into the dimples or annular grooves 40, and pimples orcorrugations 46 are thus formed in the wall of the tube 20, which fix itin the holder 22 and secure the insert 36 in the bourdon tube 20.Finally, a hole is drilled axially through the passage 26 and the holder22 and transversely through the wall of the bourdon tube 20 into theside insert 36 until it intersects with the axial bore 38 andcommunication is established between the passage 26 and the interior ofthe bourbon tube 20 via the bore 38.

FIGS. 3 and 4 show another form of the insert 36. In this insertcommunication between the passage 26 in the holder 22 and the interiorof the bourdon tube 20 is established by an axial channel 48 formed inthe peripheral surface of the insert. Moreover, the end of the insert 36remote from the inside of the bourdon tube 20 is formed with a flange 50which fixes the insert and hence the bourdon tube 20 in the holder 22.In order to prevent the measuring fluid from escaping from the end ofthe bourdon tube 20through a clearance gap that may be present betweenthe outside of the tube and the plastic body of the holder 22, aflexible washer 52 may be interposed between the end of the tube 20 andthe inside of the flange 50.

In the modified embodiment shown in FIG. 5 the end of the insert facingthe interior of the bourdon tube 20 is provided with a nozzle-likedivergent opening 54 at the end of its central axial bore 38. Thisdivergent portion 54 causes the pressure of the measuring fluid to pressthis end of the insert 36 against the inside of the wall of the bourdontube 20, thus improving fixation and the seal between the tube 20 andits holder 22. This effect will be the more pronounced the higher themeasured pressure inside the bourdon tube.

Moreover, in FIG. 5, the end 56 of the bourdon tube 20 is pinchedtogether to form a flattened beak. This creates a further seal forclosing the interior of the bourdon tube. The pinched end 56 may also beadhesively secured, soldered, welded or made tight by other means.Pinching the end of the tube laterally widens the cross-section in themanner also shown in FIG. 8 in a similar embodiment. The formation ofthis beak also improves fixation of the bourdon tube 20 inside the bodyof the holder 22.

In FIG. 6 the flattened beak 56 of the bourdon tube 20 is additionallyprovided with an upward terminal bend to improve its sealing andlocating effect. Moreover, FIG. 6 also illustrates another possible wayof fixing the insert 36 in the bourdon tube 20. This consists in theprovision of a pair of crimps 58 in the bourdon tube, so disposed thatthe end of the insert 36 facing the interior of the bourdon tube 20 willbear against these crimps or the corrugation.

The forms of the insert 36 that have already been described withreference to FIGS. 2 to 6 and that will yet be described with referenceto FIGS. 9 and are especially necessary when injection molding aroundthe ends of delicate bourdon tubes, such as are used for measuringpressures up to 60 kilopond/sq.cm. In the case of more robust bourdontubes the insert may be omitted. Instead, the necessary strength may beimparted to the bourdon tube 20 by pinching together its end to form abeak 56, as illustrated in the embodiments according to FIGS. 7 and 8.As already mentioned by reference to FIGS. 5 and 6 the flat beak 56 maybe additionally sealed and secured with an adhesive, by soldering,welding, or the like. Besides its sealing and locating effect theformation of a beak is a stabilizing step which prevents deformation ofthe end of the tube during the molding process.

In FIG. 7 an elastic sleeve 60 embraces the bourdon tube near its end.Particularly when there is shrinkage of the bourdon tube holder 22, thissleeve serves to maintain a seal and fixation between the bourdon tube20 and its holder 22. Instead of an elastic sleeve 60, as shown in thedrawing, the outside of the end of the bourdon tube 20 can be coatedwith an elastic bonding or sealing compound.

The several embodiments shown in FIGS. 2 to 8 of the connection betweenthe bourdon tube and the holder are merely illustrative. The describedfeatures could be employed simultaneously in different combinations.Thus FIG. 9 shows an insert 36 which is provided with a flange 50according to the embodiment shown in FIG. 3, as well as with dimples orgrooves 40 as in the embodiment in FIG. 2, and with a divergent mouth 54as in the embodiment according to FIG. 5.

Such an insert may additionally be fitted with the washer 52 of FIG. 3,and it may be secured in the end of a bourdon tube in a press fit, bymeans of an adhesive or by soldering or welding, and the outside of theend of the tubemay be provided with a sleeve as shown in FIG. 8 or withsome other elastic bonding or sealing compound.

FIG. 10 shows yet another arrangement of the connection between abourdon tube 20 and its holder 22. In this instance the insert 36 whichotherwise roughly has the shape already shown in FIG. 9 lacks a flange50, instead of which it has a transverse extension 62 which contains thepassage 26, and which thus provides the communicating channel throughthe entire holder 22. In addition tothe sealing washer 52 described withreference to FIG. 3 another sealing washer64 may be arranged to embracea shoulder 66 formed on the angular extension 62. Besides forming a sealthe two washers 52 and 64 also serve to locate the angular extension 62at both its ends.

FIG. 1 1 shows an embodiment of the proposed bourdon tube in which ametal tube 68 affording the passage 26 is soldered, welded or otherwiseattached to the bourdon tube 20 at 70. The passage 26 formed by themetal tube 68 communicates with the interior of the bourdon tube 20through a hole 72 already previously provided in the tube. As describedabove, the end 56 of the bourdon tube 20 is pinched together and forms abeak which imparts to the tube a given degree of stability of shape.However, for the reasons that have already been given the arrangement inFIG. llll issuitable only for more rugged types of bourdon tubes.

Finally, FIG. 12 shows another embodiment of the invention whichsomewhat resembles that described with reference to FIG. 11. In FIG. 12the end of the bourdon tube 20 is inserted into a transverse bore 73 ina metal core 74 to which it is attached at 76 by an adhesive bond,solder, a weld seam or some other means. The transverse bore 73communicates with an axial passage 26 through the metalcore 74. In thearrangement according to FIG. 12, the bourdon tube 20 is sufficientlywell supported by the metal core 74 and the joint at 76 to reduce therisk of deformation of the tube during injection molding. Thisembodiment is therefore also suitable for more delicate types of bourdontubes.

In the various arrangements that have been described the penetration ofthe plastic material into the bourdon tube during the injection moldingoperation is prevented by the end of the tube being closed either by thepresence of the insert or by being flattened to a beak fonnation, orboth.

An alternative procedure which fulfills the same purpose as thepreviously described devices, i.e. which simultaneously seals the end ofthe bourdon tube against penetration of the plastic material duringmolding and prevents the end of the tube from being damaged bydeformation, consists in first filling the bourdon tube with a solubleor meltable filler which, after the completion of the injection moldingprocess, can be removed by being dissolved or melted out of the tube.Such a filler may be wax which is removed by melting, or a solublesubstance such as a detergent powder I which can be flushed out.

According to the invention the injection mold is so designed that theplastic holder 22 and the terminal fitting 24 can both be molded aroundthe respective ends of the bourdon tube 20 at the same time in oneoperation. In a manner analogous to the steps taken for joining one endof the bourdon tube to the holder, the other end of the tube which is tobe provided with the terminal fitting 24 is likewise closed with acorresponding insert or by pinching together the end before theinjection molding process begins. The simultaneous molding of holder andterminal fitting in one injection mold has the advantage of saving timebesides ensuring that the two terminal elements will automatically formexactly predetermined reference points for the measuring system (28, 30in FIG. I). In case of need, other elements, such as a casing, parts ofthe measuring head 30, the scale 32 and the like can be produced in thesame molding operation. Instead of injection molding, which is thepreferred method of production, the bourdon tube may also be insertedinto or encast in or adhesively bonded with a plastic holder. Theplastic may be of the polyacetal type.

As already indicated, the inventionis not intended to be limited inscope to bourdon elements in the strict sense. It is also applicable toelements comprising expanding tubes that are helically, spirally orotherwise bent, and that have circular, oval or other crosssections.

What is claimed is:

1. A tubular spring element for use in measuring instruments comprisinga plastic holder having a central passage, a tubular spring having afirst end and a second end, said spring being fixed in said holder atsaid first end, a terminal fitting at said second end for closing saidsecond end, means for providing communication between the interior ofthe tubular spring communicating with and said passage means for sealingthe spring to the holder including a seal between the first end of thespring and said holder, sealing being effected by injection moulding ofthe holder about the first end of the tubular spring.

2. The element according to claim 1, further including means, at saidfirst end, for preventing deformation of the tubular spring during theinjection molding of the pastic holder.

3. The element according to claim 2 wherein said means for preventingdeformation also anchors the tubular spring in the holder.

4. The element according to claim 2, characterized in that the means forpreventing deformation comprises an insert within said tubular spring atsaid first end, said insert having a bore which communicates with theinterior of said tubular spring and with said passage.

5. The element according to claim 4 wherein said insert is rigid.

6. The element according to claim 4 wherein said bore is an axial bore.

7. The element according to claim 4 characterized in that the insert ispress fit into the first end.

8. The element according to claim 7 further including means for fixingthe insert in said first end.

9. The element according to claim 8 wherein said means for fixingcomprises a crimp in that portion of the tubular spring adjacent thefitted insert, an adhesive bond, a solder point, or a weld.

10. The element according to claim 7, characterized in that the insertwithin the tubular spring extends into the spring to a point outside ofthe holder.

11. The element according to claim 4 characterized in that the inserthas a flange which projects beyond the cross-section of the tubularspring.

12. The element according to claim 11, further including a sealingwasher bearing against the flange on the insert and against the firstend.

13. The element according to claim 4, characterized in that the tubularspring has a crimp in that portion of the spring adjacent the fittedinsert and the insert has a complementary dimple or an annular groove inits external peripheral surface.

14. The element according to claim 4 characterized in that the inserthas a divergent opening which communicates with the bore of the insertand with the interior of the tubular spring.

15. The element according to claim 14 characterized in that at least apart of the insert at said divergent opening is outside the holder.

16. The element according to claim 4 further including a depression inthe outer surface of the tubular spring at a point in the spring lyingoutside the holder, said depression forming a corresponding projectionon the inner surface of the tubular spring, said insert bearing againstsaid projection.

17. The element according to claim 4 characterized in that a portion ofthe insert extends beyond the first end of the tubular spring into atleast a part of the passage in the holder.

18. The element according to claim 17 characterized in that the portionof the insert extends through substantially the entire length of saidpassage.

19. The element according to claim 2 characterized in that the means forpreventing deformation comprises a portion of said first end of thetubular spring which is pinched together flat and laterally widened toform a beak.

20. The element according to claim 19 characterized in that the beak hasa bent end.

21. The element according to claim 2 further including an elastic sleeveembracing at least a portion of the length of said tubular spring withinthe holder.

22. The element according to claim 3 characterized in that the first endof the tubular spring is anchored in the holder and is externally coatedwith an elastic adhesive bonding or sealing compound.

23. The element according to claim 2 characterized in that said meanscomprises a removable filler material disposed within the portion of thetubular spring within the holder.

24. The element according to claim 2 wherein said means for preventingdeformation comprises a metal core having a second bore therethrough,said core being disposed within and along the axis of the centralpassage of the holder, the first end communicating with the second bore.

25. The element according to claim 1 characterized in that the terminalfitting is of plastic and said plastic fitting is injection moldedaround the second end of the tubular spring to close said second end.

III =0 1 i 0

1. A tubular spring element for use in measuring instruments comprisinga plastic holder having a central passage, a tubular spring having afirst end and a second end, said spring being fixed in said holder atsaid first end, a terminal fitting at said second end for closing saidsecond end, means for providing communication between the interior ofthe tubular spring communicating with and said passage means for sealingthe spring to the holder including a seal between the first end of thespring and said holder, sealing being effected by injection moulding ofthe holder about the first end of the tubular spring.
 2. The elementaccording to claim 1, further including means, at said first end, forpreventing deformation of the tubular spring during the injectionmolding of the pastic holder.
 3. The element according to claim 2wherein said means for preventing deformation also anchors the tubularspring in the holder.
 4. The element according to claim 2, characterizedin that the means for preventing deformation comprises an insert withinsaid tubular spring at said first end, said insert having a bore whichcommunicates with the interior of said tubular spring and with saidpassage.
 5. The element according to claim 4 wherein said insert isrigid.
 6. The element according to claim 4 wherein said bore is an axialbore.
 7. The element according to claim 4 characterized in that theinsert is press fit into the first end.
 8. The element according toclaim 7 further including means for fixing the insert in said first end.9. The element according to claim 8 wherein said means for fixingcomprises a crimp in that portion of the tubular spring adjacent thefitted insert, an adhesive bond, a solder point, or a weld.
 10. Theelement according to claim 7, characterized in that the insert withinthe tubular spring extends into the spring to a point outside of theholder.
 11. The element according to claim 4 characterized in that theinsert has a flange which projects beyond the cross-section of thetubular spring.
 12. The element according to claim 11, further includinga sealing washer bearing against the flange on the insert and againstthe first end.
 13. The element according to claim 4, characterized inthat the tubular spring has a crimp in that portion of the springadjacent the fitted insert And the insert has a complementary dimple oran annular groove in its external peripheral surface.
 14. The elementaccording to claim 4 characterized in that the insert has a divergentopening which communicates with the bore of the insert and with theinterior of the tubular spring.
 15. The element according to claim 14characterized in that at least a part of the insert at said divergentopening is outside the holder.
 16. The element according to claim 4further including a depression in the outer surface of the tubularspring at a point in the spring lying outside the holder, saiddepression forming a corresponding projection on the inner surface ofthe tubular spring, said insert bearing against said projection.
 17. Theelement according to claim 4 characterized in that a portion of theinsert extends beyond the first end of the tubular spring into at leasta part of the passage in the holder.
 18. The element according to claim17 characterized in that the portion of the insert extends throughsubstantially the entire length of said passage.
 19. The elementaccording to claim 2 characterized in that the means for preventingdeformation comprises a portion of said first end of the tubular springwhich is pinched together flat and laterally widened to form a beak. 20.The element according to claim 19 characterized in that the beak has abent end.
 21. The element according to claim 2 further including anelastic sleeve embracing at least a portion of the length of saidtubular spring within the holder.
 22. The element according to claim 3characterized in that the first end of the tubular spring is anchored inthe holder and is externally coated with an elastic adhesive bonding orsealing compound.
 23. The element according to claim 2 characterized inthat said means comprises a removable filler material disposed withinthe portion of the tubular spring within the holder.
 24. The elementaccording to claim 2 wherein said means for preventing deformationcomprises a metal core having a second bore therethrough, said corebeing disposed within and along the axis of the central passage of theholder, the first end communicating with the second bore.
 25. Theelement according to claim 1 characterized in that the terminal fittingis of plastic and said plastic fitting is injection molded around thesecond end of the tubular spring to close said second end.